19.5 million viewers watched former FBI Director James Comey's Congressional testimony on 10 different US broadcast and cable television networks.

Former FBI Director James Comey's dramatic testimony before Congress on Thursday brought big audiences to US broadcast and cable networks, drawing about 19.5 million viewers, according to data from the Nielsen ratings agency.

That is a large audience for a daytime event, though it is short of the 30.6 million who tuned in to President Donald Trump's inauguration in January.

Nielsen's estimates include people watching on 10 US broadcast and cable television networks, but not those who streamed the congressional hearing on computers or mobile phones. Broadcasters interrupted regular programming to air nearly three hours of live coverage of Comey's remarks about his dealings with Trump before the president fired him in May.

The tally was impressive for an event that aired during the work day, when typically only events like state funerals and royal weddings draw mass viewing, said Jeffrey Jones, director of journalism's Peabody Awards and a professor at the University of Georgia. Jones was traveling and saw crowds gathered around TVs in Atlanta's airport to watch the hearing.

"Politics is quite often about theater and drama, and this was a classic showdown," he said. "People wanted to see it for themselves."

The television ratings do not include the many people who watched the highly anticipated hearing during at bars and restaurants or online through TV network apps or on social media sites.

The year's most-watched television event is football's Super Bowl, which attracted 111 million viewers in February. Game Three of basketball's NBA Finals pulled in about 20 million for its primetime broadcast on Wednesday.

During his appearance, Comey told the Senate Intelligence Committee that he believed Trump dismissed him to try to undermine a Federal Bureau of Investigation probe into possible collusion between Trump's 2016 presidential campaign team and Russia.